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1. Field
The methods and systems relate to semiconductor wafer processing, and more particularly to load lock apertures used in wafer processing.
2. Background
A large variety of materials can be used in the production of wafer cassettes, with particular materials being used to meet various goals including particulate control, static discharge reduction, weight, cost and dimensional stability. While a particular material choice may achieve its goal, it may have negative effects on other aspects of wafer processing. For example, plastic cassettes or photoresist on a wafer can exhibit many structural properties that make their use desirable, however they may have negative effects on the vacuum needed during wafer processing.
In addition, the photoresist on wafers can present a large gas load to the vacuum system. High and repeatable vacuum may be required in ion implanters during wafer processing to assure correct dose repeatability and uniformity as well as to maintain throughput of the implanter or tool, i.e., the number of wafers that may be processed over a given time. In adopting new cassette or photoresist materials, such as polycarbonate, in order to improve particulate control and/or dimensional stability, end users of such equipment may not realize that the vacuum system, including the load lock and isolation valve, may need to be reconfigured to maintain tool performance.
In a typical semiconductor wafer processing system, a wafer cassette, with wafers that may or may not have photoresist masked onto them, may be loaded into a load lock station in preparation for transferring the wafers to a processing, or implant, chamber. A load lock vacuum pump can be used to bring down the pressure in the load lock to a level consistent with the implant chamber vacuum. The load lock may then be opened to the implant chamber and the wafer transferred into the chamber. In some instances, opening the load lock may result in a pressure burst in the chamber, such that a vacuum recovery period may be required to return the pressure within the chamber to levels acceptable for implanting, i.e., levels where the desired process parameter, such as implant dose repeatability, uniformity, energy contamination and/or tool throughput, can be achieved. Thus, the time to first implant, i.e., the time from the start of load lock pump down until implanting may be started can include a load lock pump down time and a process chamber vacuum recovery time.
A load lock isolation valve can be used to isolate the load lock from the implant chamber during load lock pump down. Opening of the valve can allow access to the load, or wafer in the load lock. The valve can be configured to provide an opening through which the wafer may pass. Such openings can be rectangular, circular, or ellipsoid, and may typically extend well beyond the width of the wafer in the load lock and may also extend to a greater height than the full height of the wafer and pick used to handle the wafer. As an example, the opening for a VIISta 810 HP implanter, as manufactured by Varian Semiconductor Equipment Associates of Gloucester, Mass., may be approximately 7 cmxc3x9733 cm, while the wafer that passes through the opening may have a diameter of 30 cm and may have a thickness of less than 1 mm. When opened, the relatively large opening can produce a correspondingly large pressure burst in the implant chamber. The process chamber vacuum recovery time may correspondingly be extended, thus adversely impacting the throughput of the implanter, as well as the quality of the implant.
According to the systems and methods described herein, an apparatus in combination with a load lock of an ion implanter comprises a cover adjacent a slot of the load lock, the cover defining an aperture to provide access to a load within the load lock. The cover covers a portion of the slot so as to reduce the opening between the load lock and an implant chamber of the ion implanter and thus reduce a pressure burst between the load lock and the implant chamber. In one embodiment, the cover may cover a portion of an opening between two chambers to reduce the pressure burst between the chambers. The cover may define an aperture between the two chambers that can maintain access for movement of a load between the two chambers.
The depth of the cover, as measured in a direction normal to slot, or opening, may be maximized to provide further reduction in the pressure burst. The apparatus may be removably attached to the load lock or the implant chamber, or to one of the two chambers. The size of the aperture may be reduced to the minimum required to maintain clearances for handling the wafer or load between the load lock and the implant chamber, or between the two chambers. Thus, the method for reducing a pressure burst between two chambers, such as a load lock and an implant chamber, may comprise covering a slot of the load lock with a cover having an aperture therethrough, minimizing a size of the aperture to maintain access to a load for moving the load between the load lock and the vacuum chamber and maximizing a depth of the cover measured in a direction normal to the slot of the load lock.